• The outermost “skin” of Earth with variable thickness. – Thickest under mountain ranges (70 km – 40 miles). – Thinnest under mid-ocean ridges (3 km – 2 miles). • The Mohorovičić discontinuity or “Moho” is the lower boundary. – Separates the crust from the upper mantle. – Discovered in 1909 by Andrija Mohorovicic. – Marked by a change in the velocity of seismic P waves.